New York passed a gradual emancipation law in 1799. The law stated that any children of slaves born after the enactment of the law would be born free, not as slaves, and that all slaves would be freed in 1827 regardless. So there was time for Van Buren, who was born in 1782, to own some slaves.
Grant, however, is a trickier case. Grant was born in 1822 in Ohio. Ohio was a part of the Northwest Territory, in which slavery had always been illegal. While he was married to the daughter of a Missouri slaveowner, as far as I am aware, Grant never actually lived in Missouri. But I imagine that during his time in the Army he probably lived in states where slavery was permitted and his wife probably brought some slaves along with her as servants for the household. If Grant owned any of these slaves, it was likely only through his marriage that he did so and not for having bought them himself. |